Exceptional intensities of light are also experienced in the natural world. If sunlight can be focused to cause materials to burn, imagine the effect it will have on a retina. And as the retina has been a product of evolution, so has retinal destruction. On this subject, angelfish become Hell’s angelfish when territory is at stake.
Angelfish live in the clear surface waters of the Amazon. They have flattened bodies with silver skin, similar to a mirror. When one fish invades another’s territory, the defender leaves the shelter of reeds to do battle. Battle stance is a tilted position in the water column, with the aim of firing sunlight into the eyes of the opponent. Like Roman shields, the strong Amazonian sunlight can be concentrated into a narrow beam and directed precisely. In fact both fish in this combat take up their positions in the open water, fine-tuning their lines of fire by adjusting the tilt of their bodies. Light flashes through the water like the lasers of Star Wars battles. The stakes are high. A direct hit in the eye can lead to the bursting of blood vessels and an increase in heart and breathing rates. A fish defeated in this manner is at best temporarily stunned and at worst killed. Either way, the battle is over. This is a fish living in waters where sunlight is at its most intense, and it has adapted. Acting on this strong selection pressure, it has evolved precision mirrors.
But what exactly is light? I am about to launch into the history of this key question. The question can in fact be divided into smaller inquests, the answers to which are staggered throughout the history books. Such a fundamental element of optics, however, may at first seem irrelevant to a book on evolution, even one addressing colour in nature. So why not simply provide the answer and spare the history lesson? There are clues to the cause and purpose of colour in nature throughout the historical accounts of scientific enterprise, and even throughout accounts of artistic and military endeavour. Human ingenuity and artistic expression have often converged on natural selection where colour is concerned.
In this chapter we will observe a specific animal and ask two questions: ‘What causes that colour?’ and ‘What is the purpose of that colour?’ We will move through a list of animal species, providing different answers to these questions. It will emerge that there are many possibilities in each case, but the triumphs and even tribulations of early scientists, artists and military tacticians can help narrow down the list of suspects.
As others did before him, Leonardo da Vinci strove for an explanation of light in the fifteenth century. He did, however, see things a little differently from his predecessors. Leonardo began to doubt the philosophers of his time, who believed that light was something emitted from the eye, returning on reflection from an object - the object observed. Light, Leonardo believed, was comparable to sound, and both travelled through air or water as a ‘tremor’. By this he implied a signal which is spread via a sequence of disturbances in the air or water - he was describing a wave. Throwing two stones into a river, and observing the corresponding sets of concentric waves break each other up, Leonardo wondered if light behaved in a similar way.
Leonardo became distracted and turned his attention from light to everything in the cosmos. He suggested that ‘everything was propagated by means of waves’. He should have stuck with light. At least Leonardo came to the conclusion that light was a property of the sun. From now on philosophers could think of light in terms of waves, albeit in their simplistic form.
The wave idea was taken a stage further by, and often credited to, both Christiaan Huygens and René Descartes. In 1664 Descartes described what happens to light as it passes through raindrops. He concluded that internal reflection caused the effect of a rainbow. But light was in theory only white at this stage in history. And so, consequently, was the predicted rainbow. Descartes also believed the propagation of light was instantaneous. He was to be proved wrong on both accounts.
Later in the seventeenth century, the French mathematician Pierre de Fermat breathed new life into Leonardo’s ideas that nature always acts by the shortest paths, and that light did travel at a finite speed. And according to Fermat, light travels at different speeds in water and air.
At around this time, the twenty-two-year-old Isaac Newton was discharged from Cambridge, interrupting a Bachelor of Arts degree to escape the Great Plague, which was making its way towards his university from London. The two years that followed saw perhaps the most creative display of individual genius in the history of science. At his home in Woolsthorpe, Newton formulated the binomial theorem and the differential and integral calculus in mathematics; the unification of celestial mechanics; the theory of gravity in astronomy; and . . . the theory of colour in optics. In what came to be called his experimentum crucis, Newton split light into a spectrum of colours using a prism. Then he passed each ‘colour’ through a second prism to demonstrate that further fractionation was not possible. Newton had shown that sunlight was the combination of the complete colour spectrum, and no more. Now Descartes’ rainbow could have its colours.
Newton did not have a strong view about the nature of light. In fact he favoured the idea that light consisted of particles, where the particles of different colours had either different speeds or masses. But Newton found no time to test this notion with his usual high degree of mathematical exactitude. It was Huygens’ wave theory of light that ultimately triumphed (although today we consider that all particles can behave like waves, and vice versa).
In 1690, Newton’s contemporary Christiaan Huygens stated categorically that each point in a wavefront is the source of new waves with the same frequency of oscillation. Wavelets can be cancelled out by other wavelets, travelling in a different direction, like the opposing ripples travelling from Leonardo’s two stones. But in the absence of obstacles, the wave progresses forwards.
Figure 3.1 Newton’s own drawing of his experimentum crucis. Unfortunately he lacked the artistic genius of Leonardo.
Another Victorian curiosity
The Victorians of the nineteenth century were handed all of this knowledge. They knew that sunlight contained waves of different wavelengths, and that each wavelength could be converted to a different colour by the eye (colour does not exist in the environment, only in the mind - this will be discussed in Chapter 6). But the Victorians had precision apparatus at their disposal, and on the characterisation of light they finished off what Leonardo had begun. That is, they finished it off for the purpose of this book (apologies to Planck and Einstein).
The early Victorian English physicist Thomas Young found that any colour could be obtained by combining only three different colours - blue, green and red, a useful concept that became important to science and television. Then Young discovered polarisation. When a wave travels along a guitar string, the displacement of the string is sideways. If a narrow slit is introduced into the path of this ‘wave’, the wave will continue only if the displacement is parallel to the slit. If it is not parallel, the wave will be reflected and will travel back on itself. Light behaves similarly - it is a transverse wave. Polaroid sunglasses approximate a slit to reduce light transmission. If a beam of light contains waves with different directions of displacement, or polarisations, only those parallel to ‘slits’ in the lens material will pass through. The light passing through each lens is said to be polarised.
Meanwhile, Victorian scientists were tackling another conundrum - the speed of light. Previously, the tiny displacement of stars in the sky caused by the Earth’s orbital motion around the sun was exploited to reach a surprisingly accurate value. But in the nineteenth century French and Polish scientists aimed to measure the speed of light directly, which required ingenuity and Victorian high technology. They conducted experiments where a rotating mirror was illuminated by a lamp, creating pulses of light. A second, static, mirror was placed at great distance in one direction. This reflected one light pulse back towards the rotating mirror. As the rotation speed was varied, the returning light struck the rotating mirror at slightly different angles. At one angle only, the light was reflected back
towards the lamp. Using the rotation speed of the mirror, and the distances and angles involved, the speed of light was calculated, quite accurately, as 186,355 miles per second. So light takes about eight minutes to reach us from the sun. This fact became united with the work of Maxwell.
The Scottish physicist James Clerk Maxwell became most famous for his theory of the electromagnetic field. To cut a very long story short, Maxwell found that electric particles in a medium such as air are displaced from their normal positions by the action of an electric field.
Maxwell came to realise that electric particles in his experiments were displaced through the medium in wave form. But he was also able to calculate the speed at which these waves travelled - it was the same as that measured for the speed of light! Eureka! Maxwell had discovered that light is in fact electromagnetic waves. That is, they have an electrical component and a magnetic component - waves with perpendicular displacements. And in the 1880s the German physicist Heinrich Hertz confirmed Maxwell’s theory with some ingenious experiments. But how does all this relate to the most famous Victorian science book of all, On the Origin of Species? Evolution has been subjected to the principles of optics, too.
Pigments
Travelling home on the Manly ferry one night, within Sydney Harbour, I observed Young’s theory of colour mixing in action. Part of the harbour is fringed with skyscrapers of varying heights, but all displaying their company names in neon lights. The lights are reflected from the water, forming a mirror image. But I noticed that some colours in the reflections were absent from the neon signs. Ripples in the water surface were mixing the reflected images of different buildings, including their neon signs. Where red and blue signs lay above each other on the horizon, I saw only a purple reflection.
This principle of ‘effectual colour mixing’ - as opposed to simply mixing different coloured paints on the palette - was a favourite of the French Impressionists towards the end of the Victorian era. Camille Pissarro’s painting Peasants’ Houses shows clearly a peasant passing through a garden gate, in front of a row of country cottages. Move closer to the painting and the scene becomes pixillated. Suddenly the houses disappear to become a collage of red, blue, green and yellow streaks. At distance the red and blue merge to reveal the purple shadows of chimneys - the red and blue streaks can no longer be separated by the eye. Adjacent red and blue streaks fall into the same pixel of our eye’s picture. And convergence exists in nature.
The Atlas moth grows to the size of a standard dinner plate. Its huge wings incorporate patterns of mustard and grey. This colour derives from pigments in the scales. Pigments, like those in artists’ paints and our clothes, are molecules that absorb certain wavelengths in white light. These wavelengths are no longer available to vision, but the remaining wavelengths in the sun’s spectrum are reflected from, or transmitted through, the pigment system. These are the wavelengths we see. And this is the commonest cause of colour in animals and plants - they contain pigments.
Actually, the mustard and grey Atlas moth has no mustard or grey pigments in its wing scales. Place the wing under a microscope and the grey areas become a mixture of black and white scales, and the mustard areas a mixture of brown and yellow scales. Examining the moth’s pigments at another level, in mustard we have two different types of colour - saturated and unsaturated. The yellow is saturated - that is, it contains the wavelengths for yellow and that’s all. This is saturated in the Newtonian sense. If a slit is introduced into the path of the spectrum separated by Newton’s prism, so that only the yellow part can pass through, we have a saturated yellow. Brown, on the other hand, is an unsaturated red. If the slit is moved through the spectrum so that only the red passes through, and that red is diluted by the addition of a faint white light, then brown is the colour observed. So brown is said to be unsaturated because it contains a broad range of wavelengths.
Most of the colours on the Great Barrier Reef are the result of pigments. It’s great to see such amazing colour and understand why it is so. One of the aims of this chapter is to explain how this understanding comes about; with a basic understanding of the cause of colour, one can pass through any environment and explain the hues of all its inhabitants. Although there are many other ways to appear coloured, as will be revealed later in this and other chapters, each mechanism has its own unique signature in terms of optical effects. Pigments can cause an animal, or part of an animal, to be coloured, but this colour is not the most dazzling type. Also the colours caused by pigments do not change with the direction of viewing, or when the animal itself moves. This is because pigments disperse or reflect wavelengths equally in all directions. They will thus look the same from every direction, over a complete hemisphere. And because we see only a very small cone of that hemisphere at any one time, because of the small size of our eyes, then we can receive only a small portion of the wavelengths present in the sunlight. If we possessed eyes the size of footballs, pigments, especially when close up, would appear much brighter. So the light we see is much dimmer than that of the original illumination - sunlight. And as we move further from the animal in view, the cone of light detected becomes smaller and the light dimmer. Eventually it will fade to below the limits of detection. Consider a landscape fading in the distance.
Back on the Barrier Reef, as described at the beginning of this book, the cuttlefish’s brown ink contains a pigment released into the water. While following the cuttlefish, the corals I passed over had as great a diversity of colour as forms. The saturated reds, yellows and oranges looked the same from all directions, indicating that pigments were behind them all. The sponges covered the complete spectrum, and like the reds of anemones, lobsters and starfish, they bore the saturated coloured effects of pigments. The purples and browns of sea urchins indicated unsaturated colours. But as I followed the cuttlefish around them, again their colours did not change, indicating pigments all the same. Then, as I mentioned previously, something happened to the colour of my guide. The otherwise brown and white cuttlefish turned red . . . then green.
Take a close look at a colour TV screen. When it is switched on, clusters of blue, green and red ‘sub-dots’ are distinguishable. Each of these sub-dots continually, and independently, becomes brighter and dimmer as the overall picture on the screen changes successively. This, again, is Young’s colour mixing in action. Black and white photographs in newspapers are constructed from regularly spaced black dots on a white background, just like the shades of grey achieved from the black and white scales of Atlas moth wings. The size of each dot determines the shade of grey in its particular region. The picture on a TV screen is constructed from dots, too. But here a comparable dot is made up of three sub-dots - one green, one blue and one red. And by changing the brightness (rather than size) of each sub-dot, the overall dot can appear any colour of the spectrum. So as a yellow tennis ball flashes across the grass court on the TV screen, different combinations of sub-dots glow. When the ball is over a dot, the green and red sub-dots light up, while the blue is off, to produce yellow. As the ball passes, the red sub-dot also turns off to leave green. And a yellow wave of colour travelling across a green cuttlefish works in the same way. But how can this be? Pigments produce permanent colours; they cannot suddenly change. The leopard, for instance, cannot change its pigmented spots. It was the Victorians, again, who made sense of this paradox, although not at the first attempt.
In 1802, Tom Wedgwood, son of the potter Josiah Wedgwood and uncle of Charles Darwin, took one of the first forms of a photograph. He painted leather or paper with a solution of silver nitrate, which is sensitive to light. He placed leaves on top and exposed the apparatus to sunlight for about half an hour. The light turned the exposed silver nitrate to silver metal, which reappeared, and the shape of the leaves emerged as pale silhouettes. A negative had been made, albeit in black and white. So of course colour photography became the next great Victorian goal, one eventually achieved by James Clerk Maxwell.
Prior to Maxwell’s accomplishmen
t, the nineteenth-century scientist Otto Wiener believed the colour breakthrough lay with compounds of silver chloride that react with different wavelengths of light. The new compounds formed at the end of the reaction would have the colour corresponding to the catalysing wavelength. Wiener also thought that organic substances, such as those found in animals, could possess a similar property. Then came a theory of adaptive camouflage. A caterpillar, Wiener argued, might vary its colour to match a changing environment because its skin ‘photographs that environment by means of the sensitive compounds of its own tissues’. A nice idea, but pure fiction.
The eminent Victorian naturalist Henri Milne Edwards made amends in 1848. Like Aristotle, and philosophers, scientists and poets since, Milne Edwards was intrigued by the chameleon. Chameleons change their colour dramatically. The big question is, ‘How?’ Milne Edwards realised the answer lay not with any chemical change in the skin, but with the mechanical distribution of pigments. This was a breakthrough.
In The Blink Of An Eye Page 11
